A) Field of the Invention
The present invention relates to a solid state image pickup device and its driving method, and more particularly to a solid state image pickup device having a broad optical dynamic range and its driving method.
B) Description of the Related Art
Solid state image pickup devices, typically CCD image sensors, have been developed heretofore by placing the most important issue on high integration and high sensitivity of a constituent element, pixel. A solid state image pickup device having one million pixels or more is not rare nowadays. High sensitivity also makes remarkable progress due to the development of various techniques. Application to digital cameras has achieved the performance of resolution and sensitivity superior to that of a conventional silver salt film.
However, as to the dynamic range (optical dynamic range) of exposure amount, a solid state image pickup device is far inferior to a conventional silver salt film. A narrow optical dynamic range causes various phenomena including so-called white blank areas: an area uniformly painted solid white in the details of a white wedding dress photographed with a camera flash and an area unnaturally painted solid white in the highlighted portions of the tip of a nose and cheeks; and an inability of rendering a background scene outside a window in a photographed indoor scene.
In a photodiode of a solid state image pickup device, electric charges are optically excited and accumulated in proportion to an exposure amount. There is a maximum value of charges capable of being accumulated in each photodiode. When the charge amount reaches the maximum value, no more charges will be accumulated. The characteristics of a photodiode show a linear region where the amount of charges is proportional to the exposure amount and a saturated region where the amount of charges will not increase more than the maximum value. The higher the sensitivity, the output voltage saturates at the lower exposure amount. In order to achieve a broad optical dynamic range, the lower the sensitivity, the better. However, as the sensitivity is lowered, it becomes difficult to photograph a relatively dark subject.
FIG. 5 is a graph showing the photoelectric conversion characteristics of a solid state image pickup device. The abscissa represents an exposure amount of incidence light on a solid state image pickup device (not on each photoelectric conversion element). The ordinate represents an output voltage generated by the solid state image pickup device in accordance with electric charges accumulated in photoelectric conversion elements. A saturated output voltage is a voltage at which the output voltage saturates even if the exposure amount is increased. At the saturated output voltage, the maximum amount of electric charges is accumulated in photoelectric conversion elements. A saturated exposure amount is an amount at the time when the output voltage takes the saturated output voltage.
Two sensitivity characteristics HS and LS are shown having the same saturated output voltage Vsat.
A curve LS represents a low sensitivity with a small increase in an output voltage relative to a unit exposure amount. An exposure amount reaching the saturated output voltage Vsat, i.e., a saturated exposure amount SEL, is large, resulting in a broad optical dynamic range DL.
A curve HS indicates a large increase in an output voltage (sensitivity) relative to a unit exposure amount, representing a high sensitivity. A saturated exposure amount SEH reaching the saturated output voltage Vsat is low, resulting in a narrow optical dynamic range DH. As seen from FIG. 5, a high sensitivity narrows the optical dynamic range.
FIG. 6 is a plan view illustrating an example of conventional techniques realizing a broad optical dynamic range. On a semiconductor chip, a plurality of photodiodes 51, 52 are disposed in a square or tetragonal matrix shape. Along each photodiode column, a vertical charge transfer device (VCCD) 55 is disposed. The photodiodes 51 and 52 have the same area and the same saturated output voltage. One end of each vertical charge transfer device 55 is coupled to a horizontal charge transfer device (HCDD) 56. An output of the horizontal charge transfer device 56 is supplied to an output circuit 57.
A light shielding film is formed above the photodiodes to define an opening for each photodiode. An opening 53 above the photodiode 51 in an odd row is broad, whereas an opening 54 above the photodiode 52 in an even row is narrow. An amount of light passing through the opening changes with the area of the opening. At the same exposure amount, the light amount received by the photodiode 52 having the narrow opening 54 is smaller than that received by the photodiode 51 having the broad opening 53.
The sensitivity of the photodiode 52 with the narrow opening 54 is therefore lower than that of the photodiode 51 with the broad opening 53. The photodiode 51 has the high sensitivity characteristics HS shown in FIG. 5 and the photodiode 52 has the low sensitivity characteristics LS shown in FIG. 5.
The sensitivity of the high sensitivity photodiode 51 is represented by R1 and that of the low sensitivity photodiode 52 is represented by R2. Since both the photodiodes have the same saturated output voltage, the saturated exposure amount of the low sensitivity photodiode 52 is R1/R2 times the saturated exposure amount of the high sensitivity photodiode.
Image signals are read to an external from the high sensitivity photodiode 51 and low sensitivity photodiode 52 and thereafter synthesized. An image of a small light amount dark area in a photographed scene can be generated mainly from a signal from the high sensitivity photodiode 51, whereas an image of a large light amount bright area in the photographed scene can be generated mainly from a signal from the low sensitivity photodiode 52. With this structure, an image pickup device can be obtained which has a high sensitivity and a broad optical dynamic range.
The structure shown in FIG. 6 forms one pixel by one high sensitivity photodiode 51 and one low sensitivity photodiode 52. Since one pixel is constituted of two photodiodes, the number of effective pixels is reduced to a half and the resolution is lowered to a half.
Another approach for obtaining a broad dynamic range is to photograph the same scene twice with the same photodiodes. High sensitivity photographing and low sensitivity photographing are performed during different signal accumulation time durations, a long exposure time and a short exposure time, respectively. Two images photographed during different time durations are synthesized so that a high sensitivity and broad optical dynamic range can be obtained similar to the structure shown in FIG. 6. Although this approach is effective for a standstill subject, a reproduced image of a moving subject is corrupted because two different photographing times are incorporated. These techniques described above are disclosed, for example, in Japanese Patent Laid-open Publications Nos. HEI-09-116815 and HEI-09-252107.